The invention relates to an NMR(=nuclear magnetic resonance) probe head with an NMR coil system disposed around a preferably vertical z axis and a device for the radial centering relative to the z axis of a longitudinal sample vial closed at one end and filled with a substance to be measured, wherein the NMR coil system is fixed radially relative to the NMR probe head with respect to the z axis, wherein at least two centering devices are provided spaced from each other in the axial direction of the z axis with passage openings for the sample vial for the centering of the sample vial in the radial direction only, of which at least a first centering device is disposed in the z direction above the NMR coil system, and wherein all centering devices are fixed radially with respect to the z axis.
Such an assembly is known, for example, from U.S. Pat. No. 6,563,317 B2 (=Reference [1]).
NMR spectroscopy is a powerful process of instrumental analysis. RF (radio-frequency) pulses are irradiated into a measurement sample, which is located in a strong, static magnetic field, and the RF response of the sample is measured. By this means, the information is retrieved integrally across a specific region of the measurement sample, the so-called active volume.
As a rule, the measurement sample consists of a cylindrical sample vial that contains the solid or liquid substance to be measured. The vial can have a circular, oval, or rectangular cross section. It is closed at least at the end with which it first enters the probe head. The closed end is typically rounded.
Typically, the sample vial is located in a spinner. It can either be permanently connected to the spinner or positioned freely in the spinner. The sample vial and the spinner are transported from outside the magnet into the probe head by means of a transport system. In the description below, it will be assumed that the insertion opening is located at the top of the probe head and that the sample vial is inserted into the probe head from above. However, it is also conceivable to insert the sample vial into the probe head from below into an opening intended for that purpose. This case is analogous to that described above and, for the sake of clarity, will not be explicitly described. If the sample vial is in the measuring position, the spinner is located inside the turbine. The sample vial can be rotated by means of the turbine.
In the measuring position, the sample vial is surrounded by one or more NMR coils. The innermost NMR coil has an inside diameter that is only slightly larger than the outside diameter of the measuring probe because the fill factor and thus the sensitivity of the NMR coil depends on the inside diameter and is reduced as the inside diameter increases.
Various fragile probe head components can be disposed around the sample vial when it is in the measuring position. This might, for example, be the innermost NMR coil or also a glass tube that holds the innermost NMR coil. Defined in general terms, we refer to an endangered region around the sample vial in the measuring position. During transport of the sample vial into the measuring position, collision with such fragile components, which are located in the endangered region, usually results in time-consuming repair of the probe head.
In reference [1], the sample vial is centered in the measuring position by two centering devices with a radial action, which are disposed axially above and below the coil. The centering devices are rigidly connected to the holder device of the receiver coil. In addition, positioning is provided in the axial direction only. This can be performed either below the receiver coil by means of an end stop or above the receiver coil inside a modified spinner with the aid of a fastening sleeve.
The centering devices of the prior art (see references [1] and [2] including the sources cited therein) are solely concerned with ensuring that the position of the sample vial is controlled precisely and reliably in the measuring position. However, precise guidance of the sample vial is not achieved all the time during transport into the measuring position. If the sample vial is still located above the measuring position, the lower end of the sample vial, in particular, can move radially a relatively large distance away from the probe head axis. As a result, the components of the probe head that are located in an endangered region may be damaged during transport into the measuring position.
Even a slight probability of such damage occurring during insertion of the sample vial into the probe head is very disadvantageous if the NMR spectrometer is operated as a high-throughput analytical system. Using automatic sample vial changers and filling robots it is namely possible to perform a large number of measurements on different samples in a very short time. In this case, even the combination of a slight probability of damage and a large number of sample changes can result in an unacceptable failure rate.
The object of this invention is therefore to modify an NMR probe head of the type defined above using as simple technical measures as possible, so that the disadvantages given above are largely eliminated, wherein the transport of the sample vial from entry into the probe head until it reaches the measuring position can be controlled in such a way that probe head components cannot be damaged by the sample vial in an endangered region inside the probe head.
In order to keep down tines as low as possible, in particular, when using an automatic sample vial changer, a very high degree of safety during the individual operations is sought. Contact by the sample vial with the probe head in the endangered region must therefore be fully excluded with certainty.
Another object of this invention is to permit fast exchange of the sample vial without risking any considerable reduction in the lifetime of the probe head.